1. Place prawns, onion and spices in a food processor or blender and process until smooth. Stir in rice flour, oil and coriander. Cover and refrigerate mixture for 1 hour. Using wet hands, roll mixture into small balls, then roll in seeds. Refrigerate for 1 hour.
2. To make sauce, blend cornflour with stock in saucepan. Stir in tamarind sauce, soy sauce, lime juice and coriander. Bring to the boil, stirring constantly, reduce heat and simmer, uncovered, for 5 minutes.
3. Heat oil in a large saucepan. Cook balls until golden and cooked through. Serve balls with sauce.